Although choice of a measure should be geared to a specific population for a specific purpose, if I was hard-pressed to identity one objective measure for use with children and adolescents, that meets your criteria of being relialbe, valid, easy to administer for repeated measures, useful in outpatient and inpatient settings, and has clinically relevant information, I would select the Achenbach Child Behavior Checklist. It has parent and teacher versions, as well as a youth self-report version, and I routinely use it as a baseline and follow-up measure as treatment progresses. It has various administration and scoring modes, all computer, all paper, paper-administered and computer scored, even scantron format. I find it really easy to use and very helpful. There is much data to support its utility, especially for its basic premise of identifying a clinical sample. Further, it looks at behavior in broad categories, Internalizing and Externalizing symptoms, and more discretely with its subscales. Hope this is helpful to you. For more specific diagnoses, by the way, I use other scales, especially ADHD, OCD and anxiety, and social skills.